The present invention relates to a magnetron unit and, more particularly, to an improvement in a magnetron unit having means for compensating the magnetic field to be applied to the interaction space inside the magnetron unit.
Generally, a magnetron unit has a pair of permanent magnet members. The temperature of the permanent magnet members is raised mainly by heat generated at the anode in the operation of the magnetron unit. With the temperature rise of the permanent magnet members, the magnetic energy of the permanent magnet members decreases. Alnico magnets and ferrite magnets, which have been widely used as the permanent magnet members of the magnetron unit, have reversible temperature coefficients of residual flux density of approximately -0.02%/.degree.C. and -0.2%/.degree.C. respectively. As seen from comparing these reversible temperature coefficients, the magnetic energy of a ferrite magnet depends more on temperature than that of an alnico magnet. Accordingly, in a magnetron unit with a pair of ferrite magnet members, the intensity of the axial magnetic field generated in the interaction space decreases greatly with an increase in temperature within the anode cylinder. This greatly changes the performance of the magnetron unit. A magnetron device employing the magnetron unit generally uses a leakage transformer for increasing the power source impedance to make the anode current uniform. With such a magnetron unit, when the intensity of the magnetic field inside the interaction space is reduced by the increase in temperature, the anode current may increase due to the characteristics of the magnetron unit. However, such an increase of the anode current is suppressed by the leakage transformer so that the anode current is kept substantially constant. However, the anode voltage supplied by the leakage transformer is decreased so that the electric power fed to the magnetron unit is reduced. As a result, power of the microwaves generated by the magnetron unit decreases or becomes unstable. Depending on the characteristics of the leakage transformer, the anode current which tends to increase cannot be maintained substantially constant, so that an excessive current flows to the leakage transformer and the leakage transformer becomes burned out.